U.S. Pat. No. 3,113,176 issued on Dec. 3, 1963 to T. L. Doktor, G. Parker and L. A. Weber describes a semiautomatic full-duplex data transmission system specifically adapted to the communication of asynchronous teletypewriter signals between terminals connected through the switched telephone network. The telephone voiceband is divided into a lower and an upper band, designated respectively F1 band and F2 band. Frequency-shift keying (FSK) is employed in each band such that the F1 band has a spacing frequency of 1070 Hz and a marking frequency of 1270 Hz and the F2 band has a spacing frequency of 2025 Hz and a marking frequency of 2225 Hz. The terminals are mirror images of each other and either terminal can originate calls to which the other can respond. A direction-of-transmission convention is adopted such that the originating terminal transmits in the low F1 band and receives in the high F2 band and the answering terminal transmits in the high F2 band and receives in the low F1 band.
Each teletypewriter terminal, according to the patented disclosure, is provided with an attendant telephone subscriber set capable of dialing up calls through telephone central offices. Assuming the presence of attendants at two terminals between which teletypewriter messages are to be exchanged, the originating attendant goes off-hook, waits for dial tone, and dials up the number assigned to the answering terminal. The telephone central office, in response to the dialed digits, performs the conventional switching action to extend the calling terminal to the called terminal.
The answering attendant, hearing the ringing signal, goes off-hook and operates an answer key. A timer is started to bridge a silent interval during which the off-hook signal is returned to the telephone central office. The answering terminal then transmits the high-band marking frequency of 2225 Hz to notify the originating terminal of the completion of the connection.
The originating terminal recognizes the 2225 Hz answering tone and responds with the low-band marking frequency of 1270 Hz.
Both terminals are now in the clear-to-send condition and both attendants can return the telephone handsets to the on-hook condition. Teletypewriters at both terminals can send their messages in an FSK mode. Calls are terminated by either terminal by transmitting a continuous spacing signal in the appropriate frequency band.
It is also known to equip either or both terminals for automatic calling and answering without the presence of an attendant.
A commercially available data set providing the above-described capabilities is designated Western Electric Type 103.
In U.S. Pat. No. 3,937,882 granted to J. A. C. Bingham on Feb. 10, 1976 there is disclosed a full-duplex two-wire communication system for synchronous serial data transmission over voiceband channels. The channel is frequency-divided into an upper and a lower band for directional purposes as in the low-speed asynchronous application, but the synchronous timing permits scaling up the data rate by a factor of four. Quadrature amplitude modulation on fixed carrier waves near the centers of the upper and lower bands is suggested as preferable. Start-up line protocol appears to comprise manual origination followed by the exchange of a high-band steady tone burst and a low-band phase-shift-keyed (PSK) signal between answering and origination stations. The Bingham system contemplates a single synchronous transmission speed.
It is an object of this invention to provide a dual speed, full-duplex data transmission system on two-wire analog transmission facilities connecting geographically separated terminals.
It is another object to provide a full-duplex data transmission system at not less than two distinct bit rates over the switched telephone network.
It is a further object to provide a dual-speed, full-duplex data transmission system in which the answering station automatically adapts to the speed of the originating terminal.
It is a still further object to provide a dual-speed, dual-format, full-duplex data transmission system in which the answering station automatically adapts to the speed and format of the originating station.
It is yet another object of this invention to provide a consistent line protocol in a dual-speed, dual-format, full-duplex data transmission system which protocol automatically instructs the answering station to adopt the correct responsive condition and assures the originating station that its instructions are being followed.
It is an additional object of this invention to provide a line protocol which is at once compatible with an existing low-speed data transmission system and yet allows data transmission at an alternative speed and format.